U.S. patent application number 10/002128 was filed with the patent office on 2002-11-21 for ink jet recording device capable of detecting defective nozzle with high signal-to-noise ratio.
Invention is credited to Kida, Hitoshi, Kobayashi, Shinya, Satou, Kunio, Shimizu, Kazuo, Yamada, Takahiro.
Application Number | 20020171717 10/002128 |
Document ID | / |
Family ID | 18858490 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020171717 |
Kind Code |
A1 |
Satou, Kunio ; et
al. |
November 21, 2002 |
Ink jet recording device capable of detecting defective nozzle with
high signal-to-noise ratio
Abstract
When positively charged minute ink droplets 608 from a defective
nozzle impact and cling on a negatively charged deflector electrode
320, the positive charge at a side of a condenser 609 opposite to
the side connected to the deflector electrode 320 flows to the
ground via a FET 618 of a photo-coupler 610. As a result, the
electric discharge occurs by the amount equivalent to the charging
amount of the minute ink droplets 608 clinging on the deflector
electrode 320. At this time, because a switching signal 606 is "1",
the ON resistance of the photo-coupler 610 is large, and the ON
resistance of the FET 620 of the photo-coupler 612 is small.
Accordingly, the discharge due to the charged minute ink droplets
608 is detected as a large detection voltage and amplified at an
operation amplifier 613 at a high rate. Because the charger voltage
of the condenser 609 is static and has no noise, even when the
detection output 615 is amplified at the high rate, the noise
during the detection is greatly suppressed.
Inventors: |
Satou, Kunio;
(Hitachinaka-shi, JP) ; Yamada, Takahiro;
(Hitachinaka-shi, JP) ; Kida, Hitoshi;
(Hitachinaka-shi, JP) ; Kobayashi, Shinya;
(Hitachinaka-shi, JP) ; Shimizu, Kazuo;
(Hitachinaka-shi, JP) |
Correspondence
Address: |
WHITHAM, CURTIS & CHRISTOFFERSON, P.C.
11491 Sunset Hills Road, Suite 340
P.O. Box 9204
Reston
VA
20190
US
|
Family ID: |
18858490 |
Appl. No.: |
10/002128 |
Filed: |
December 5, 2001 |
Current U.S.
Class: |
347/77 |
Current CPC
Class: |
B41J 2/09 20130101 |
Class at
Publication: |
347/77 |
International
Class: |
B41J 002/09 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2000 |
JP |
P2000-392512 |
Claims
What is claimed is:
1. An ink jet recording device comprising: a head formed with a
nozzle and selectively ejecting an ink droplet from the nozzle; a
deflecting means for deflecting a flying direction of the ink
droplet ejected from the nozzle, the deflecting means including a
first electrode and a second electrode; a mode selecting means for
selecting one of a first mode and a second mode; an applying means
for applying a direct voltage to the first electrode and another
direct voltage to the second electrode throughout the first mode
and the second mode, the direct voltage differing from the another
direct voltage; and a detection means for detecting a quantity of
electricity relating to an electric discharge flowing through the
first electrode in the second mode.
2. The ink jet recording device according to claim 1, further
comprising a control means for controlling the head to eject ink
droplets in the first mode and not to eject ink droplets in the
second mode, wherein the detection means detects the quantity of
the electric discharge caused by ink droplets clinging onto the
first electrode.
3. The ink jet recording device according to claim 2, wherein the
head is formed with a nozzle line including a plurality of nozzles
and selectively ejects ink droplets from the nozzles.
4. The ink jet recording device according to claim 1, wherein the
applying means includes a condenser that applies the direct voltage
to the first electrode.
5. The ink jet recording device according to claim 1, wherein the
applying means includes a battery that applies the direct voltage
to the first electrode.
6. The ink jet recording device according to claim 1, wherein the
applying means includes a battery and a condenser, the battery
applying a first voltage of the direct voltage to the first
electrode in the first mode, the condenser applying a second
voltage of the direct voltage to the first electrode in the second
mode.
7. The ink jet recording device according to claim 6, further
comprising a control means for controlling the head to eject ink
droplets in the first mode and not to eject ink droplets in the
second mode, wherein the detection means detects the quantity of
the electric discharge caused by ink droplets clinging onto the
first electrode.
8. The ink jet recording device according to claim 7, wherein the
mode selecting means selects one of the first mode, the second
mode, and a third mode, and the control means controls the head and
the deflecting means in response to a print data to form an image
corresponding to the print data on a recording medium in the third
mode.
9. The ink jet recording device according to claim 1, further
comprising an amplifying means for amplifying the quantity of
electricity at an amplifying rate, and a rate setting means for
setting the amplifying rate to a first rate in the first mode and
to a second rate in the second mode, the first rate being lower
than the second rate.
10. A control method for controlling an ink jet recording device,
comprising the steps of: a) selecting a first mode; b) applying a
direct voltage to a first electrode and another direct voltage to a
second electrode throughout the first mode and a second mode, the
direct voltage differing from the another direct voltage; c)
ejecting an ink droplet from a nozzle of an ink jet head in the
first mode; d) switching from the first mode to the second mode;
and e) detecting a quantity of electricity relating to an electric
discharge flowing through the first electrode.
11. The control method according to claim 10, wherein the quantity
of the electric discharge caused by ink droplets clinging onto the
first electrode is detected in the step e).
12. The control method according to claim 10, wherein the direct
voltage applied to the first electrode in the step b) is applied
from a battery.
13. The control method according to claim 10, wherein the direct
voltage applied to the first electrode in the step b) is applied
from a condenser.
14. The control method according to claim 10, wherein in the step
b) a first voltage of the direct voltage is applied from a battery
in the first mode, and a second voltage of the first voltage is
applied from a condenser in the second mode.
15. The control method according to claim 14, wherein the quantity
of the electric discharge caused by ink droplets clinging onto the
first electrode is detected in the-step e).
16. The control method according to claim 15, further comprising
the steps of f) switching from the second mode to a third mode for
forming an image corresponding to print data on a recording
medium.
17. The control method according to claim 10, further comprising
the steps of g) amplifying the quantity of electricity at a first
amplifying rate in the first mode, and h) amplifying the quantity
of electricity at a second amplifying rate greater than the first
amplifying rate in the second mode.
18. The control method according to claim 10, wherein the steps a)
through e) are repeatedly performed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an ink jet recording device
having a monitor function for monitoring ink droplet generating
condition.
[0003] 2. Related Art
[0004] There has been proposed a line scanning type ink jet
printer, capable of printing images on an elongated uncut recording
sheet at a high printing speed. This type of printer includes a
head having a plurality of nozzles and an elongated width covering
over the entire width of the recording sheet. When printing images,
ink droplets are ejected from the nozzles, charged, and deflected,
and then impact on the recording sheet that is being fed at a high
speed in its longitudinal direction. The impact positions of the
ejected ink droplets on the recording sheet are controlled based on
a recording signal. By controlling the impact positions of the ink
droplets and the feed of the recording sheet, a desired image is
formed on the recording sheet.
[0005] There are two types of line scanning type ink jet printer.
One includes a continuous ink jet head, and the other includes an
on-demand ink jet head.
[0006] Although, the printer with the on-demand ink jet head is
slow in printing speed compared with the printer with the
continuous ink jet head, the on-demand ink jet head requires a
simple ink system, and so is well suited for general-purpose
high-speed printer.
[0007] When a nozzle of ink jet printers becomes defective, a part
of an image corresponding to the defective nozzle may be left out
or may have an unevenness in ink density, resulting in degradation
of image quality. Therefore, in order to maintain a high quality of
images, it is necessary to monitor the ink ejection condition of
each nozzle.
[0008] Japanese Patent-Application Publication No. SHO-61-53053
discloses an ink jet printer having a monitor function for
monitoring ink droplet generation. After an ink-droplet-charging
signal is generated to charge ink droplets for a certain period of
time, a charged-amount-detection signal is detected for a certain
period of time so as to detect charging condition of the ink
droplets. A changeable amplifying means amplifies the
charged-amount-detection signal at an amplification rate. An
amplification-rate-control-signal generation circuit generates and
outputs an amplification-rate-control signal to control the
changeable amplifying means to change the amplification rate.
Specifically, the amplification-rate-control signal controls the
changeable amplifying means to set to a lower amplification rate
when the ink-droplet charging signal is being generated, and to a
higher amplification rate when the charged-amount-detection signal
is being detected. In this way, the charged amount, i.e., charging
condition of ink droplet, is detected while preventing a detection
error, because electrical noise is not amplified other than when
the charged amount-detection signal is being detected.
SUMMARY OF THE INVENTION
[0009] However, in the above printer, because a pulse-shaped high
voltage signal is used as the ink-droplet charging signal, its
influence is reflected in the charged-amount detection signal,
which is a weak signal, so the signal-to-noise ratio (SNR) becomes
small.
[0010] It is an object of the present invention to overcome the
above problems, and also to provide an ink jet recording device
capable of detecting the ink droplet generation condition with high
SNR.
[0011] In order to achieve the above and other objective, there is
provided an ink jet recording device including a head formed with a
nozzle and selectively ejecting an ink droplet from the nozzle, a
deflecting means for deflecting a flying direction of the ink
droplet ejected from the nozzle, the deflecting means including a
first electrode and a second electrode, a mode selecting means for
selecting one of a first mode and a second mode, an applying means
for applying a direct voltage to the first electrode and another
direct voltage to the second electrode throughout the first mode
and the second mode, the direct voltage differing from the another
direct voltage, and a detection means for detecting a quantity of
electricity relating to an electric discharge flowing through the
first electrode in the second mode.
[0012] There is further comprising a control method for controlling
an ink jet recording device. The control method comprises the steps
of a) selecting a first mode, b) applying a direct voltage to a
first electrode and another direct voltage to a second electrode
throughout the first mode and a second mode, the direct voltage
differing from the another direct voltage, c) ejecting an ink
droplet from a nozzle of an ink jet head in the first mode, d)
switching from the first mode to the second mode, and e) detecting
a quantity of electricity relating to an electric discharge flowing
through the first electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the drawings:
[0014] FIG. 1 is a plan view showing a configuration of an ink jet
printer according to an embodiment of the present invention;
[0015] FIG. 2(a) is a time chart of a print-mode/detect-mode
switching signal;
[0016] FIG. 2(b) is a time chart of ejection signal;
[0017] FIG. 2(c) is a time chart of voltage applied to a first
deflector electrode;
[0018] FIG. 2(d) us a time chart of voltage applied to a second
deflector electrode;
[0019] FIG. 2(e) is a time chart of a detection signal;
[0020] FIG. 2(f) is a time chart of charging-mode/detection-mode
switching signal;
[0021] FIG. 2(g) is a time chart of a condition of a switch;
[0022] FIG. 2(h) is a time chart of a condition of a
photo-coupler;
[0023] FIG. 2(i) is a time chart of a condition of a
photo-coupler;
[0024] FIG. 3 is a plan view of components, partially indicated in
a block diagram, of the ink jet printer;
[0025] FIG. 4 is a magnified view of component of FIG. 3;
[0026] FIG. 5(a) is an explanatory view showing charging-deflection
control signals applied to the charger electrodes of the ink jet
printer;
[0027] FIG. 5(b) is an explanatory view showing PZT driving signals
applied to nozzles and corresponding deflection amounts of ink
droplets; and
[0028] FIG. 6 is an explanatory view showing dots formed on a
recording sheet.
PREFERRED EMBODIMENT OF THE PRESENT INVENTION
[0029] Next, an ink jet printer 1 according to an embodiment of the
present invention will be described while referring to the attached
drawings.
[0030] The ink jet printer 1 shown in FIG. 1 has a print mode and a
detect mode. The print mode is for printing operation for forming
images on a recording medium. The detect mode is for detecting any
nozzle that has became defective. The detect mode is automatically
set when a main power of the ink jet printer 1 is turned ON, or
once every hour or once every 1,000 pages printing, for example.
Needless to say, the detect mode can be manually set as desired or
can be set both manually and automatically.
[0031] The detect mode includes a charging mode for charging
operation and a detection mode for detection operation. Typically,
the charging operation and the detection operation together require
1 ms. Performing these two operations twice (2 ms) improves
detection precision.
[0032] First, the printing operation in the print mode will be
described while also explaining a configuration of the ink jet
printer 1.
[0033] The ink jet printer 1 of the present embodiment forms an
image on an elongated uncut recording sheet 100 of FIG. 3.
[0034] Specifically, the elongated uncut recording sheet 100 has a
width in a first direction A and a length in a second direction B
perpendicular to the first direction A, and is transported in the
second direction B at a predetermined speed. The ink jet printer 1
forms dots on scanning lines 110 (FIG. 4) on the recording sheet
100 at a dot density of DS so as to form a dot image on the
recording sheet 100 at a high speed.
[0035] As shown in FIGS. 3 and 4, the ink jet printer 1 includes a
recording head 200, which includes a plurality of head modules 210
arranged in the first direction A and a frame 220 for supporting
the head modules 210. Each head module 210 has the same
configuration, and is formed with a nozzle line 211 extending in a
third direction C. The nozzle line 211 includes N nozzles 230
aligned in the third direction C at a pitch of Pn, and each nozzle
230 has a nozzle hole 231 opened to a nozzle surface of the head
module 210. The recording head 200 is positioned so that the nozzle
surface faces a recording surface of the recording sheet 100 while
keeping the distance of 1 mm through 2 mm therebetween.
[0036] As shown in FIG. 4, each nozzle 230 has the same
configuration and has an ink chamber 232 with the nozzle hole 231,
an ink supply port 233 for introducing ink into the ink chamber
232. The head module 210 is formed also with a manifold 234 for
distributing ink to the ink supply port 233 of each nozzle 230. The
ink chamber 232 is provided with a piezoelectric element 235, such
as PZT, serving as an actuator. The piezoelectric element 235
changes a volume of the ink chamber 232 when applied with recording
signals.
[0037] In the present example, scanning lines 110 extend in the
second direction B and have a line density DS of 300 dpi in the
first direction A. The angle .theta. of the third direction C with
respect to the second direction B is approximately 11.3 degrees
(=tan.sup.-1(1/5)). The nozzle-hole pitch Pn is {fraction (2/300)}
(sin(1/5)).sup.-1 inches, i.e., approximately 0.034 inches. The
number N of nozzles 230 is 96.13 head modules 210 are used, which
is sufficient for covering over the entire width of recording head
200.
[0038] The ink jet printer 1 also includes a plurality of pairs of
deflector electrodes 310, 320, an electrode substrate 330, a
deflection-control-signal generating unit 400, and an ink-ejection
control-signal generating unit 500. Each pair of electrodes 310,
320 are provided between the recording sheet 100 and the recording
head 200 and sandwich a corresponding one of the nozzle lines 211
therebetween. The electrode 310 serves as a positive-polarity
deflector electrode, and the electrode 320 serves as a
negative-polarity deflector electrode. The electrodes 310, 320 are
connected to a positive-polarity deflector-electrode terminal 341
and a negative-polarity deflector-electrode terminal 342,
respectively, which are provided on the electrode substrate
330.
[0039] The deflection-control-signal generating unit 400 is for
applying deflection control signals to the deflector electrodes
310, 320, and includes a charging-signal generating unit 410, a
positive-polarity deflector voltage supply 421, a negative-polarity
deflector voltage supply 422, a positive-polarity biasing circuit
431, and a negative-polarity biasing circuit 432.
[0040] The charging-signal generating unit 410 generates charging
signal voltage for charging ink droplets. The positive-polarity
deflector voltage supply 421 and the negative-polarity deflector
voltage supply 422 generate and output deflector voltages. The
positive-polarity biasing circuit 431 and the negative-polarity
biasing circuit 432 superimpose the charging signal voltage onto
the deflector voltage, thereby generating charging-deflecting
control signals S1, S2, which are applied to the electrodes 310,
320, respectively.
[0041] The ink-ejection control-signal generating unit 500 includes
a recording-signal generating circuit 510, a timing-signal
generating circuit 520, a PZT-driving-pulse generating circuit 530,
and a PZT driver circuit 540. The recording-signal generating
circuit 510 generates pixel data of images based on input data or
test pattern data. The timing-signal generating circuit 520
generates a timing signal for determining operation timings of the
ink jet printer 1. The PZT-driving-pulse generating circuit 530
generates a PZT driving pulse for each nozzle 230 based on the
pixel data and the timing signal. The PZT driving pulse is for
controlling the proper ink ejecting timing. The PZT-driver circuit
540 amplifies the PZT driving pulse to a signal level sufficient
for driving the piezoelectric element 235, and outputs the
amplified PZT driving pulse to the piezoelectric element 235 of
each nozzle 230, so that an ink droplet is ejected from the nozzle
230 at a proper timing. The timing-signal generating circuit 520
also generates print-mode/detect-mode switching signals 605,
charging-mode/detection-mod- e switching signals 606, and ejection
signals 607 as described later.
[0042] FIG. 5(a) shows the charging-deflecting control signals S1
and S2 applied to the electrodes 310 and 320, respectively. FIG.
5(b) shows PZT driving pulses Sa through Sd for each nozzle 230 and
also corresponding ink-droplet deflection amounts Ca through Cd.
FIG. 6 shows dots recorded on the recording sheet 100. Details will
be described next.
[0043] When the electrode 310 for a positive polarity is applied
with the charging-deflecting control signals S1, a deflector
voltage of +H and a charger voltage are applied to the electrode
310. Similarly, when the electrode 320 for a negative polarity is
applied with the charging-deflecting control signals S2, a
deflector voltage of -H and the charger voltage are applied to the
electrode 320. The magnitude of the charger voltage changes every
time period T in a stepped manner among 0V and .+-.Vc. As a result,
a charger electric field for charging ink droplets and a deflector
electrostatic field for deflecting the charged ink droplets are
generated.
[0044] The ink held in the recording head 200 is electrically
connected to the ground, i.e., has 0V. Therefore, at the time of
when the ink droplet 130 is about to be ejected from the nozzle
hole 231, the charger voltage is applied between the ink droplet
130 and the electrodes 310, 320. Because the ink has an excellent
conductivity of lower than several hundreds .OMEGA.cm, at the time
of when the ink droplet 130 separates from the rest of the ink, the
ink droplet 130 is charged by an amount corresponding to the
charger voltage applied at that moment. Then, the charged ink
droplet 130 flies toward the recording sheet 100. Before impacts on
the recording sheet 100, the ink droplet 130 is deflected by the
deflector electrostatic field by a deflection amount in proportion
to the charged amount toward a forth direction D perpendicular to
the third direction C (FIG. 2).
[0045] Referring to FIG. 4, an ink droplet 130A ejected from a
nozzle hole 231A can impact on any scanning lines 110.sub.n+1 to
110.sub.n+5 depending on its deflection amount, and therefore can
form any dot 140.sub.AN+1 to 140.sub.AN+5. Similarly, an ink
droplet 130B ejected from a nozzle hole 231B can impact on any
scanning lines 110.sub.n+3 to 110.sub.n+7 by deflection, and an ink
droplet 130C from a nozzle hole 231C is deflected to impact on any
scanning lines 110.sub.n+5 to 110.sub.n+9. That is, the ink
droplets 130A, 130B, 130C from three different nozzle holes 231A,
231B, and 231C can impact on the single scanning line 110.sub.n+5.
Also, two ink droplets from different nozzle holes can impact on
the scanning line 110.sub.n+4. The same is true for the scanning
line 110.sub.n+6.
[0046] The recording operations will be described further in more
detail. It should be noted that as described above the PZT driving
pulses Sa through Sd of FIG. 5(b) are applied to the piezoelectric
elements 235 for ejecting ink droplets 130. FIG. 6 shows dots
formed on the recording sheet 100 and projections 231A', 231B',
231C' of the nozzle holes 231A, 231B, 231C of FIG. 4. The line
segments extending perpendicular to the direction C are time
division/deflection reference lines L. The interval of the
reference lines L indicates the time interval T, the direction of
the reference lines L indicate a direction of the deflection, and
the length of the reference lines L indicates the deflection
amount.
[0047] As shown in FIGS. 5(a) and 5(b), at the time T1, the charger
voltage is .+-.0. Accordingly, an ink droplet 130A ejected from the
nozzle hole 231A at the time T1 is not charged. Accordingly, the
ink droplet 130A is not defected but flies straight, and then
impacts on, for example, a pixel 120A.sub.T1 on the scanning line
110.sub.n+3 of FIG. 6, forming a dot thereon. At a subsequent time
T2, because the PZE driving signal pulse is not applied to the
piezoelectric element 235 of the nozzle 230A, no ink droplet is
ejected at the time T2, and so no dot is formed. At the time T3,
the charger voltage is -Vc, so an ink droplet ejected at the time
T3 is deflected by an amount of -2. The ink droplet impacts on a
pixel 120A.sub.T3 on the scanning line 110.sub.n+5, and forms a dot
thereon. At the time T4, no dot is formed by an ink droplet from
the nozzle hole 231A. At the time T5, the charger voltage is
+1/2Vc, so an ink droplet ejected at the time T5 is deflected by an
amount of +1. The ink droplet impacts on a pixel 120A.sub.T5 on the
scanning line 110.sub.n+2, and forms a dot thereon. The same
operation is performed with respect to the nozzle-holes 231B, 231C,
231D and on, so that dots are formed on other pixels also as shown
in FIG. 6.
[0048] In this manner, ink droplets 130A ejected from the nozzle
hole 231A are selectively deflected and able to impact on every
pixel on the five scanning lines 110.sub.n+1 through
110.sub.n+5.
[0049] Next, the operation in the detect mode will be described
while referring to a monitoring mechanism of the ink jet printer
1.
[0050] It is assumed in this example that the nozzle 230 shown in
FIG. 1 is defective, and an ink droplet 608 that is smaller in size
than a proper ink droplet is ejected from the nozzle 230. The
nozzle 230 becomes defective for different reasons, for example,
when the nozzle 230 is clogged, when air bubbles are trapped in the
nozzle 230, or when a portion around the nozzle hole 231 is
unevenly wet with ink. In this condition, the defective nozzle is
incapable of ejecting ink, or ink droplet is ejected at an angle.
Sometimes, an ink droplet is ejected along with additional minute
ink droplets.
[0051] As shown in FIG. 1, the ink jet, printer 1 further includes
a monitoring mechanism 10 provided to each nozzle 230. The
monitoring mechanism 10 includes switches 600, 601, and 602, which
together determine the operation mode of the ink jet printer 1. For
example, the connection conditions shown in FIG. 1 of the switches
600, 601, 602 indicate that the ink jet printer 1 is in the
charging mode of the detect mode.
[0052] The switches 600 and 601 are connected to the deflector
electrodes 310 and 320, respectively, and change their connection
condition in response to the print-mode/detect-mode switching
signal 605. The switch 602 is turned ON and OFF in response to the
charging-mode/detection-mode switching signal 606. Each of the
switching signals 605 and 606 are output from the timing-signal
generating circuit 520 and takes the value of either "0" or
"1".
[0053] When the switching signal 605 of "1" is output to the
switches 600 and 601, this means that the print mode is selected,
the switches 600 and 601 connect the electrodes 310, 320 to the
deflection-control-signal generating unit 400.
[0054] When setting to the detect mode, the switching signal 605 is
switched from "1" to "0", so that the switches 600 and 601 are
switched into the connection condition shown in FIG. 1, and the
operation mode is switched from the print mode to the detect
mode.
[0055] When the switching signal 605 is switched to "0" in this
manner as shown in FIG. 2(a), the switching signal 606 is initially
set to "0" as shown in FIG. 2(f). As a result, the switch 602 is
turned ON as shown in FIGS. 1 and 2(g), and the operation mode is
set to the charging mode.
[0056] In this charging mode, that is, in the condition shown in
FIG. 1, the deflector electrode 310 is connected to the ground,
that is, set to 0V (FIG. 2(c)). On the other hand, the deflector
electrode 320 is connected to a charger voltage supply (battery)
603 via a resister 604 and the switch 602. The charger voltage
source 603 supplies a DC voltage of -V1 to the deflector electrode
320 (FIG. 2(d). At the same time, a condenser 609 is also charged
with -V1 from the charger voltage supply 603 via the resister
604.
[0057] The ejection signal 607 shown in FIG. 2(b) is output to the
piezoelectric element 235. Because the nozzle 230 of FIG. 1 is
defective as mentioned above, the minute ink droplets 608 are
ejected from the nozzle 230. At this time, the minute ink droplets
608 are positively charged by a charger electric field generated by
the deflector electrode 301 with 0V and the deflector electrode 320
with -V1.
[0058] Next, as shown in FIG. 2(f), the switching signal 606 is
switched to the value of "1", and the operation mode is switched
from the charging mode to the detection mode. As a result, the
ejection signal 607 is stopped (FIG. 2(b)), and the switch 602 is
turned OFF (FIG. 2(g)). Because no ejection signal 607 is output,
no ink droplet is ejected from the nozzle 230. Also, because the
switch 602 is turned OFF, the charged voltage of the condenser 609,
which is negatively charged during the charging mode, is applied to
the deflector electrode 320, so that the second deflector electrode
320 is maintained at -V1(V) (FIG. 2(d)).
[0059] Accordingly, the positively charged ink droplets 608 are
pulled toward the negatively charged deflector electrode 320 and
impact thereon. It should be noted that because an ink droplet in a
proper size flies at a higher speed, the positively-charged ink
droplet in a proper size do not impact on the deflector electrode
320 but reaches the recording sheet 100. However, because the
minute ink droplets 608 are slow in their flying speed, the
droplets 608 keep pulled toward the deflector electrode 320 during
both the charging mode and the detection mode and impact thereon
eventually. When the positively charged ink droplets 608 impacts
and cling on the deflector electrode 320, the negative charge of
the condenser 609 is canceled out by the positive charge of the ink
droplets 608. As a result, the positive charge at a side of the
condenser 609 opposite to the side connected to the deflector
electrode 320 flows to the ground via a field effect transistor
(FET) 618 of a photo-coupler 610. That is, the electric discharge
occurs by the amount equivalent to the charging amount of the
minute ink droplets 608 clinging on the deflector electrode
320.
[0060] The photo-couplers 610 and 612 control the electric current
flowing through light-emitting diodes (LEDs) 617 and 619 (input
side) so as to control the ON resistance of the FETs 618 and 620
(output side), respectively. The ON resistance can change from tens
.OMEGA. to hundreds M.OMEGA..
[0061] In the detection mode, the switching signal 606 is "1"as
mentioned above. Therefore, no electric current flows to the LED
617 of the photo-coupler 610, so that the ON resistance of the
photo-coupler 610 is large (FIG. 2(h)) Also, because an inverter
616 outputs a signal of "0" to the photo-coupler 612, an electric
current flows to the LED 619 of the photo-coupler 612, so that the
ON resistance of the FET 620 of the photo-coupler 612 is small
(FIG. 2(i)).
[0062] Accordingly, the discharge due to the charged minute ink
droplets 608 is detected as a large detection voltage at the both
sides of the FET portion 618, impedance-converted at the operation
amplifier 611, amplified at an operation amplifier 613 at an
amplification rate, which is determined by the resistance of the
resister 614 and the ON resistance of the FET portion 620, and so
producing a detection output 615 (FIG. 2(e)). That is, the
detection output 615 is amplified at a high rate in the detection
mode. Because the charger voltage of the condenser 609 is static
and has no noise, even when the detection output 615 is amplified
at the high rate, the noise during the detection is greatly
suppressed.
[0063] On the other hand, when the switching signal 606 is "0" in
the charging mode, the ON resistance of the photo-coupler 610 is
small, and the ON resistance of the FET 620 is large, so that the
amplification rate is small.
[0064] In this way, because stable and low noised deflector DC
voltage is used over the charging period in the charging mode to
the detection period in the detection mode, and also because the
voltage is controlled to the lower amplification rate at the
charging period and to the higher amplification rate in the
detection period, the detection output 615 with a high SNR can be
obtained.
[0065] By performing the above charging operation and detection
operations twice, the detection precision is enhanced as mentioned
above. In other words, in the detect mode, the ejection signal 607
are intermittently output, and a defective nozzle is detected based
on the discharge due to the ink droplets impacted on the electrodes
302 at the time of when the ejection signal 607 is not output. The
electrode 320 is applied with a negative voltage from the battery
603 and the condenser 609 in the detect mode.
[0066] While some exemplary embodiments of this invention have been
described in detail, those skilled in the art will recognize that
there are many possible modifications and variations which may be
made in these exemplary embodiments while yet retaining many of the
novel features and advantages of the invention.
* * * * *